U.S. patent number 8,295,151 [Application Number 12/531,558] was granted by the patent office on 2012-10-23 for method central unit, and modem in a digital subscriber line network.
This patent grant is currently assigned to Telefonaktiebolaget LM Ericsson (publ). Invention is credited to Boris Dortschy, Aldebaro Klautau, Rodrigo Bastos Moraes, Jaume Rius i Riu, Ronaldo Zampolo.
United States Patent |
8,295,151 |
Dortschy , et al. |
October 23, 2012 |
**Please see images for:
( Certificate of Correction ) ** |
Method central unit, and modem in a digital subscriber line
network
Abstract
A method, central unit, and modem for reducing crosstalk in a
Digital Subscriber Line (DSL) system. A virtual line referred to as
a ghost line is introduced in the system as a substitution for all
of the lines except a first line to induce crosstalk to the first
line. The Power Spectral Density (PSD) on the first line is
allocated to optimize against the ghost line. A modem on the first
line reports to a central Spectrum Management Center (SMC), a
measure indicating crosstalk impact from the ghost line. This
process is repeated for each line. The SMC calculates updated ghost
line parameters for each line, which reflect current crosstalk
characteristics between each modem and the remaining modems, and
sends the updated parameters to the modems. The modems then
reallocate PSDs with respect to the updated ghost line parameters
to either maximize the rate or minimize the power on their
respective lines.
Inventors: |
Dortschy; Boris (Hagersten,
SE), Rius i Riu; Jaume (Vallingby, SE),
Moraes; Rodrigo Bastos (Rio de Janeiro, BR), Klautau;
Aldebaro (Belem, BR), Zampolo; Ronaldo (Belem,
BR) |
Assignee: |
Telefonaktiebolaget LM Ericsson
(publ) (Stockholm, SE)
|
Family
ID: |
39788718 |
Appl.
No.: |
12/531,558 |
Filed: |
March 23, 2007 |
PCT
Filed: |
March 23, 2007 |
PCT No.: |
PCT/SE2007/050179 |
371(c)(1),(2),(4) Date: |
September 16, 2009 |
PCT
Pub. No.: |
WO2008/118048 |
PCT
Pub. Date: |
October 02, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100103993 A1 |
Apr 29, 2010 |
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Current U.S.
Class: |
370/201;
370/420 |
Current CPC
Class: |
H04B
3/32 (20130101); H04L 27/2601 (20130101) |
Current International
Class: |
H04J
3/10 (20060101); H04L 12/28 (20060101) |
Field of
Search: |
;370/201,254,255,420,485 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1 670 202 |
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Jun 2006 |
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EP |
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2003-264484 |
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Sep 2003 |
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JP |
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2007-067527 |
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Mar 2007 |
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JP |
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WO 98/06186 |
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Feb 1998 |
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WO |
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Other References
Tsiaflakis, P. et al. An Efficient Search Algorithm for Lagrange
Multipliers of Optimal Spectrum Balancing in Multi-User XDSL
Systems. ICASSP 2006 Proceedings. 2006 IEEE International
Conference on Acoustics, Speech and Signal Processing. May 14-19,
2006. cited by other .
Cendrillon, R. et al. Iterative Spectrum Balancing for Digital
Subscriber Lines. ICC 2005. IEEE International Conference on
Communications. Seoul, Korea. May 16-20, 2005. cited by
other.
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Primary Examiner: Cho; Hong
Claims
The invention claimed is:
1. A method for reducing crosstalk on a first line of a Digital
Subscriber Line (DSL) network, the method comprising the steps of:
introducing a ghost line into the network, the ghost line being a
substitution of the lines of the DSL network excluding the first
line; receiving by a central agent, a measurement sent by a modem
of the first line indicating the impact of crosstalk on the first
line due to the ghost line; calculating by the central agent,
updated ghost line parameters for the first line based on the
received measurement and on other measurements received from other
lines in the network for which ghost lines were introduced; and
transmitting the calculated updated ghost line parameters to the
modem of the first line such that the first modem can update power
to be allocated to the first line based on the transmitted updated
ghost line parameters.
2. The method according to claim 1, wherein the measurement
indicating the impact of crosstalk is received from a respective
modem of each line of the DSL network.
3. The method according to claim 1, wherein the measurement
indicating the impact of crosstalk includes information about
frequencies at which the modem suffers from crosstalk.
4. The method according to claim 3, wherein the measurement also
indicates a fraction of channel resources that must be allocated to
achieve a given minimum rate.
5. The method according to claim 1, wherein the step of calculating
updated ghost line parameters for the first modem includes
determining an allowed crosstalk range as a decision variable.
6. The method according to claim 5, wherein an extent of the
allowed crosstalk range depends on a value of the measurement.
7. The method according to claim 5, wherein the step of calculating
updated ghost line parameters includes maintaining previously
determined ghost line parameters when the decision variable is
within the allowed crosstalk range.
8. The method according to claim 5, wherein the step of calculating
updated ghost line parameters includes increasing a coupling
between the ghost line and the first line when the decision
variable is above the allowed crosstalk range.
9. The method according to claim 5, wherein the step of calculating
updated ghost line parameters includes decreasing a coupling
between the ghost line and the first line when the decision
variable is below the allowed crosstalk range.
10. The method according to claim 5, further comprising:
introducing in the DSL network, a weighting matrix indicating the
interference between the lines of the network; wherein the step of
determining an allowed crosstalk range as a decision variable
includes calculating the decision variable based on the
interference matrix.
11. A method performed in a modem of a first line of a Digital
Subscriber Line (DSL) network in which an initial virtual ghost
line having initial parameters is introduced as a substitution of
the lines of the DSL network excluding the first line, the method
comprising the steps of: allocating an initial power to the first
line that is optimized against the ghost line; transmitting to a
central agent of the DSL network a measurement indicating the
impact of crosstalk on the first line due to the initial ghost
line; receiving updated ghost line parameters from the central
agent as a response, wherein the updated ghost line parameters are
at least determined based on the measurement indicating the impact
of crosstalk; and updating the power to be allocated to the first
line based on the received updated ghost line parameters.
12. The method according to claim 11, wherein the step of
transmitting a measurement includes transmitting a measurement that
includes information about frequencies at which the modem suffers
from crosstalk and the fraction of channel resources that must be
allocated to achieve a given minimum rate.
13. A central unit for reducing crosstalk on a first line of a
Digital Subscriber Line (DSL) network having a plurality of lines,
wherein for each given line of the DSL network, an associated
virtual ghost line substituting for the remaining lines of the DSL
network excluding the given line is introduced, wherein the central
unit comprises: a receiver for receiving from modems of each of the
plurality of lines, respective measurements indicating the impact
of crosstalk on each line due to the line's associated ghost line;
a calculator for calculating based on the received measurements, a
decision variable for each modem and updated ghost line parameters
for each modem based on each modem's decision variable; and a
reporter for transmitting the calculated updated ghost line
parameters to each modem such that each modem in the DSL network
can update the power to be allocated to each line based on the
transmitted ghost line parameters.
14. The central unit according to claim 13, wherein the measurement
indicating the impact of crosstalk includes information about
frequencies at which the modem suffers from crosstalk and the
fraction of channel resources that must be allocated to achieve a
given minimum rate.
15. The central unit according to claim 13, wherein the calculator
for calculating updated ghost line parameters includes means for
determining as a decision variable, an allowed crosstalk range
depending on a value of the measurement.
16. The central unit according to claim 15, wherein the calculator
includes means for maintaining previously determined ghost line
parameters when the decision variable is within the allowed
crosstalk range.
17. The central unit according to claim 15, wherein the calculator
includes: means for increasing a coupling between a given ghost
line and the given ghost line's associated line when the decision
variable is above the allowed crosstalk range; and means for
decreasing a coupling between a given ghost line and the given
ghost line's associated line when the decision variable is below
the allowed crosstalk range.
18. The central unit according to claim 13, wherein a weighting
matrix indicating the interference between the lines of the DSL
network is introduced, and the calculator includes means for
calculating the decision variable also based on the interference
matrix.
19. A modem of a first line of a Digital Subscriber Line (DSL)
network in which a virtual ghost line substituting for the
remaining lines of the DSL network excluding the first line is
introduced and an initial ghost line is determined, the modem
comprising: means for allocating an initial power to the first line
that is optimized against the ghost line; a transmitter for
transmitting to a central agent of the DSL network, a measurement
indicating the impact of crosstalk on the first line due to the
ghost line; a receiver for receiving updated ghost line parameters
from the central agent as a response, wherein the updated ghost
line parameters are at least determined based on the measurement
indicating the impact of crosstalk; and a Power Spectral Density
(PSD) calculator for updating the power to be allocated to the
first line based on the received updated ghost line parameters.
20. The modem according to claim 19, wherein the measurement
includes information about frequencies at which the modem suffers
from crosstalk and the fraction of channel resources that must be
allocated to achieve a given minimum rate.
Description
TECHNICAL FIELD
The present invention relates to methods and arrangements in a
Digital Subscriber Line (DSL) network. In particular, the invention
concerns methods and arrangements for minimizing the deleterious
effect of crosstalk in a DSL network.
BACKGROUND
Digital Subscriber Lines are the most important means for
delivering high-speed Internet access. Crosstalk has been
identified as one of the main sources of performance degradation in
DSL networks. Crosstalk is the effect of electromagnetic coupling
of different lines transmitting in the same binder--the phenomenon
can be interpreted as if the signal of one line leaks into all
neighboring lines as illustrated in FIG. 1. Balancing crosstalk is
a compensating game: decreasing crosstalk by reducing transmit
power and thus increasing system performance goes typically along
with decreasing individual line performance. Crosstalk is a major
impairment for improvements in rate and reach in the network, thus
crosstalk is one of the most important limiting factor for better
service provisioning and increase in the number of users served by
the technology.
Recently, new strategies for dealing with crosstalk have been
created. Crosstalk interference in a given receiver of interest
depends basically on two factors: the transmitter Power Spectral
Densities (PSDs) of all users different than the user of interest
and the coupling function from these transmitters to the receiver
of interest. There is no possible way to manipulate crosstalk gains
in a binder, but it is feasible to design users' PSDs such that
crosstalk is minimized by still maintaining the system's data
rates, and maybe even increasing it. Strategies to optimize and
custom design the users' PSDs are referred to as Dynamic Spectrum
Management (DSM).
There are two main approaches for the DSM problem in the DSL: the
Rate Maximization Problem (RMP), often also referred to as Rate
Adaptive (RA) problem [Starr, Sorbara, Cioffi, Silverman, "DSL
Advances", Prentice Hall] and the Power Minimization Problem (PMP),
often also referred to as Fixed Margin (FM) problem [Starr,
Sorbara, Cioffi, Silverman, "DSL Advances", Prentice Hall].
Consider an N-user multicarrier system that splits the available
spectra in K tones. Let p.sub.n.sup.k be the PSD of user n on tone
k. Consider the matrix arrangement P.sub.(NxK) of all
p.sub.n.sup.k, as follows
.times. ##EQU00001##
The upper left-corner element will denote the PSD of user 1 in the
first tone. The lower right-corner element will denote the PSD of
the N-th line in tone K. One row of matrix P, which will be
referred to as P.sub.n, will represent the PSD distribution of user
n across all tones, i.e., P.sub.n=[p.sub.n.sup.1, p.sub.n.sup.2, .
. . , p.sub.n.sup.K-1, p.sub.n.sup.K]. One column of matrix P,
which will be represented as P.sup.k, will represent the PSD
allocation of all users across one tone, i.e.,
P.sup.k=[p.sub.1.sup.k, p.sub.2.sup.k, . . . p.sub.N-1.sup.k,
p.sub.N.sup.k].
One can formulate the RMP as the task of finding a given matrix P
such that the data rate of one given user (say, user 1) is
maximized while all other users in the network achieve a minimum
desired rate R.sub.n.sup.min and a limited power budget for each
user is respected. One but not exclusive formulation of the RMP
could be
.times..times..times. ##EQU00002## such that
R.sub.n.gtoreq.R.sub.n.sup.min.A-inverted.n>1;
P.sub.n.sup.tot.ltoreq.P.sub.n.sup.max.A-inverted.n in which the
rates R.sub.n.sup.min denotes the said minimum rate and
P.sub.n.sup.max denotes the said maximum power constraints.
P.sub.n.sup.tot can be determined as sum of the n-th row in
equation (0) and R.sub.n can be determined as sum of the rate on
each tone of user n in the multicarrier system.
As stated above, the main objective behind the RMP is the
optimisation of PSDs under the given set of constraints.
The objective-function of the RMP problem can be re-written as a
weighted rate-sum maximization,
.times..times..times..times..times..times..times..times..times..times..lt-
oreq..times..A-inverted. ##EQU00003## with a certain set of weights
or priorities w.sub.n of user n. By controlling the w.sub.n, one
controls how much resources (in terms of power) a line can or must
use to achieve a maximum objective. In the solution the set of
w.sub.n is uniquely determined by the minimum rates constraints and
thus no constraints are neglected. Often it is further assumed
that
.times. ##EQU00004## In practice, the right w.sub.n are not known
in advance and are (iteratively) found such that all rate
constraints are respected. In this case, these variables can be
interpreted as the amount of channel resources needed for each user
to achieve (at least) a specific minimum rate. Often the first user
should take "the maximum rest", i.e.
.times..times..times. ##EQU00005##
The interpretation of the w's is further developed, if set C=1, in
which case the w's get a proportional meaning.
The PMP can be formulated as the task of finding a set of PSDs for
all users as to minimize total power allocated in the network such
that a given set of minimum data-rates is achieved. Hence, the PMP
problem can be (non-exclusively) described as
.times..times..times..times..times..times. ##EQU00006## such that
R.sub.n.gtoreq.R.sub.n.sup.min;
P.sub.n.sup.tot.ltoreq.P.sub.n.sup.max.A-inverted.n in which the
w.sub.n has the same interpretation of weight or priority as in the
case of the RMP (see also Eq. (2)).
Four properties of the different ways to solve the RMP and the PMP
are of higher interest, i.e. complexity, centralization,
performance and required knowledge. Whereas complexity can simply
be described as number of required operations, performance is
usually described as a function of the achieved R.sub.n. Since the
achievable rates are related, it is standard procedure to look for
the extending of the rate region: the wider, the better.
Centralisation refers to the coordination between the
determinations of the PSDs for each user. In a non-centralized
schemes (usually called autonomous) the PSDs are determined without
any further knowledge of other lines (for example their PSDs or
channel information). In contrast, in a full-centralized schemes,
the knowledge about all users operations and channels are assumed
and exploited. In this case a central management is often assumed
to concentrate this knowledge and all operations. Required
knowledge is the amount of information necessary or assumed in the
different schemes to work. Complexity and performance could be
considered as a matter of "taste", centralization and required
knowledge are of immediate importance. Channel measurements are
time consuming and expensive and centralization is a key question
with respect to unbundling of lines and competition between
different service providers.
A brief description of existing algorithm follows in chronological
order.
The most representative example of a fully autonomous solution to
the DSM problem is the Iterative water filling (IWF) method
disclosed in W. Yu, G. Ginis, and J. Cioffi, "Distributed multiuser
power control for digital subscriber lines," IEEE Journal on
Selected Areas of Communications, vol. 20, pp. 1105-1115, 2002. IWF
uses the well-known water-filling solution iteratively across the
network with each user utilizing the minimum power necessary to
achieve a given minimum data-rate. It enjoys low complexity,
autonomous implementation and requires no crosstalk channel
knowledge, However, it is clearly sub-optimal in near-far
scenarios.
OSB (Optimal Spectum Balancing) demands a fully centralized system
in a central agent with complete channel knowledge. Its complexity
scales exponentially in the number of user, thus making its use for
large networks prohibitive. It assumes convexity of the rate region
and use Lagrange variables to decouple the problem across frequency
to solve a per-tone maximization to come up with optimal results
for the DSM problem. OSB is described in EP 01492261. ISB
(Iterative Spectrum Balancing) is the iterative version of OSB. It
optimally solves the RMP with smaller computational demands but
still requires centralized operation and full channel
knowledge.
SCALE disclosed in J. Papandrlopoulos and J. S. Evans,
"Low-complexity distributed algorithms for spectrum balancing in
multi-user DSL networks," in IEEE International Conference on
Communications (ICC), 2006 utilizes a convex approximation of the
original non-convex objective function and iterates through it
until this approximation is as close as possible to the original
formulation.
ASB described in J. Huang, R. Cenchillon, M. Chiang, M. Moonen,
"Autonomous Spectrum Balancing (ASB) for Frequency Selective
Interference Channels," in IEEE International Symposium Infounation
Theory (ISIT), Seattle, 2006 uses the concept of a reference line
to represent in each modem its impact on other modems. The
reference line should represent the typical victim in a binder. In
this context a victim of a line A is considered the line, which has
most performance degradation due to the crosstalk of this line A.
The reference line is used as an opponent line in a two-line
optimization scheme performed for each line separately and is
classified by its PSD, the crosstalk gain assumed from user n to
the reference line and a background noise. ASB is further
characterized by the definition of a static, pre-definition (i.e.
before the optimization is done) reference line, which is used
unchanged and being the same for all lines to be optimized.
Based on that, the following drawbacks follow: The ASB method
demands that each modem must know the reference line parameters
before all--in other words, the network needs an initial
configuration. The definition of the reference line is static and
does not take into account the dynamic nature of a network, i.e.
system changes such as new line or lines going out of the system
are not covered. The utilization and performance of the reference
line method is based on the assumption of its most advantageous
definition. It is usually unknown in advance what this definition
should really be. Rather complete channel knowledge is necessary to
even start further consideration of how a "typical" victim could
look like. In reality, this channel knowledge can, in some
situations, be imprecise or not available at all. A reference line
is not individually defined for each physical line, i.e. it is
defined the same for all lines. This must at least prevent
optimality and is probably impossible for large networks due to the
spread of relations and channel and system properties.
Due to the fact that there is only one reference line definition
and that this must be defined in advance, any change of the system
affects all lines at the same time by a re-initialization of the
reference line.
SUMMARY
The algorithms of prior art except sub-optimal IWF assume in
general perfect and full channel knowledge, which usually is not
available in practice. Also, good performance also often implies a
higher complexity. Therefore, the object of the present invention
is to achieve a near-optimal low-complexity scheme that depends the
least possible on channel knowledge.
The object of the present invention is achieved by the introduction
of a so-called ghost line. The ghost line is a fictitious line and
reflects the impact of transmission of a particular line to the
remaining ones. The central agent is responsible for adjusting the
ghost line parameters according to the present network state. The
central agent is able to adjust the ghost line parameters through
message-passing steps between the transmitting modems and the
central agent.
According to a first aspect of the present invention, a method for
reducing crosstalk on a first line of a DSL network, wherein a
ghost line being a substitution of the lines of the DSL network
excluding the first line is introduced. The method comprises the
steps of receiving from a first modem of the first line and at
least from a second modem of a second line a respective measure
(CDR) indicating the impact of crosstalk, calculating based on the
received measure (CDR) indicating the impact of crosstalk a
decision variable (.PHI.) for the first modem and ghost line
parameters for the first modem based on the decision variable
(.PHI.) for the first modem, and transmitting the calculated ghost
line parameters (G) to the modem of the first line such that the
first modem can update the power to be allocated to the first line
based on the transmitted ghost line parameters.
According to a second aspect of the present invention a method for
a modem of a first line of the DSL network is provided whereby a
ghost line being a substitution of the lines of the DSL network
excluding the first line is introduced and an initial ghost line is
determined. The method comprises the steps of allocating an initial
power to the first line that is optimized against the ghost line,
transmitting to a central agent of the DSL network a measure
indicating the impact of crosstalk. As a response updated ghost
parameters are received, whereby the updated ghost line parameters
are at least determined based on the measure indicating the impact
of crosstalk. Then the power to be allocated to the first line can
be updated based on the received updated ghost line parameters.
According to a third aspect a central unit for reducing crosstalk
on a first line of the DSL network is provided, wherein a ghost
line being a substitution of the lines of the DSL network excluding
the first line is introduced. The central unit comprises a receiver
for receiving from a first modem of the first line and at least
from a second modem of a second line a respective measure (CDR)
indicating the impact of crosstalk, a calculator for calculating
based on the received measure (CDR) indicating the impact of
crosstalk a decision variable (.PHI.) for the first modem and ghost
line parameters for the first modem based on the decision variable
(.PHI.) for the first modem, and a reporter for transmitting the
calculated ghost line parameters to the modem of the first line
such that the first modem can update the power to be allocated to
the first line based on the transmitted ghost line parameters.
According to a fourth aspect a modem of a first line of the DSL
network is provided, whereby a ghost line being a substitution of
the lines of the DSL network excluding the first line is introduced
and an initial ghost line is determined. The modem is configured to
allocate an initial power to the first line that is optimized
against the ghost line. The modem comprises a transmitter for
transmitting to a central agent of the DSL network a measure (CDR)
indicating the impact of crosstalk, a receiver for receiving
updated ghost line parameters as a response, whereby the updated
ghost line parameters are at least determined based on the measure
indicating the impact of crosstalk, and a PSD calculator for
updating the power to be allocated to the first line based on the
received updated ghost line parameters.
An advantage with the present invention is that it presents a
method to increase transmission performance in discrete-multi-tone
based transmission systems. It allows finding a solution for the
crosstalk problem associated with DSL transmission which implies
that it finds transmit PSDs for every user and tone in the network
so that data rates are maximized or power minimized on a system
level. The method is trustable and stable.
Numerical experiments show that the proposed method achieves
near-optimal performance with surprisingly low complexity and very
limited demands on a-priori channel knowledge, especially when
compared to existing solutions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates crosstalk originated in the remote terminal that
affects the transmission of the central office.
FIG. 2 illustrates an embodiment of the present invention.
FIG. 3 illustrates the crosstalk damage ratio that is used in an
embodiment of the present invention.
FIG. 4 is graph showing the rate region for the scenario
illustrated in FIG. 1.
FIG. 5 is a block diagram showing the method according to an
embodiment of the present invention.
FIG. 6 illustrates the scenario that was simulated in order to
compare the method of the present invention with methods of prior
art.
FIG. 7 illustrates the rate regions of the simulations of the
scenario of FIG. 6.
DETAILED DESCRIPTION
In this description, the rate maximization problem is focused on,
but the proposed method and arrangements are also applicable to the
power minimization problem.
The basic idea of the present invention is to provide the users
(i.e. the lines) in the network with a ghost line, i.e. a virtual
line that should reflect the damage to be caused to the remaining
users in the network, and that each user allocates the PSDs such
that the allocated PSD is optimized against this ghost line, either
according to the RMP or according to the PMP. The difference
between the ghost line and the reference line of ASB is that the
ghost line is not static. A central agent is adapted to
continuously and individually update the ghost lines by collecting
a measure indicating the impact of crosstalk from all modems. The
measure indicating the impact of crosstalk comprises preferably
information about in which frequencies the modems suffer from
crosstalk and to which extend. This measure is sent from modems and
collected by a central agent referred to as a Spectrum Management
Centre (SMC). When the central agent has received updated measures
indicating the impact of crosstalk from the modems it is then able
to calculate updated ghost line parameters, which, by further
iteration, should reflect the current crosstalk situation between
each modem and the remaining modems. The central agent subsequently
transmits the updated ghost line parameters G to all modems. Each
modem may then allocate PSDs such that its allocated PSD is
optimized with respect to the updated ghost line. This procedure is
preferably repeated for each modem. As said above this per-line
optimization step is individually performed on each line and may
have as target of maximizing the rate or minimizing the power.
In this way, the dynamic nature of DSL channels is considered and
the network adjusts itself independently of initial conditions to a
more profitable and intelligent state, in which each modem is both
aware of its rate or power requirements and also its impact on
other lines.
Thus, the present invention relates to a method and arrangements
for reducing crosstalk on a first modem 201 in a DSL network.
Turning now to FIG. 2, where an exemplarily embodiment of the
present invention is shown. The present invention relates to a
central agent 204 and modems 201-203 of a DSL network 200. The
central agent is preferably located in the central office. The
central agent 204 is provided with a minimum rate Rn min and/or a
maximum power Pn_max requirement. In accordance with the invention
a ghost line to a first line is introduced wherein the ghost line
is supposed to act as a substitution of the lines of the DSL
network excluding the first line. Ghost lines of all modems are
determined for each line individually and provided to each
corresponding individual line. Hence, the ghost line of the first
line is not presented to other lines.
An initial PSD allocation which is an optimization with respect to
an initial ghost line is determined at each modem 201-203 by the
PSD calculator 209b, 211b, 213b. The initial ghost line may be a
fixed setting or suggested by the central office. Each modem
201-203 transmits by means of a crosstalk reporter 208, 210, 212 to
the central agent 204 a respective measure CDR.sub.1, CDR.sub.2,
CDR.sub.3; indicating the impact of crosstalk for each relevant
modem, i.e. the first modem 201 and the neighbouring modems 202,
203 of the first modem. The measure may be the parameter CDR
(Crosstalk damage ratio) and additionally the parameter w.sub.n, as
explained below. The measure may be any parameter that gives
information about the crosstalk impact, and the additional
parameter may be any parameter that gives information about how
much (fractional) channel resources must be shared with the ghost
line to achieve the minimum requirements. The central agent 204
then receives the measure at a receiver 205 and determines at the
ghost line calculator 206 updated ghost line parameters as a
function of the reported information.
The updated ghost line parameter Gi comprising the updated coupling
are reported to the first modem 201 received at the receiver 209a
(211b and 213b denotes receiving means at the modems 202 and 203,
respectively) by the reporter 207 of the central agent. It should
be noted that the only ghost line parameter that is changed is the
crosstalk gain h.sub.n,G.sup.k. Based on the report the first modem
201 is arranged to allocate the PSD by the PSD calculator 209. It
should be noted that the updated ghost line parameters are
calculated for each tone for the first modem 201, and the procedure
described above for the first modem should be repeated for the
remaining modems 202, 203 in the DSL network.
A further embodiment of the present invention is explained in the
sequence and the text below:
A minimum required rate R.sub.k.sup.min is determined for each
modem n. Input is a minimum rate requirement. INPUT:
R.sub.n.sup.min.A-inverted.n.gtoreq.1 The output should be the set
of optimized PSDs one for each line which is denoted P
TABLE-US-00001 OUTPUT: P 1. Set p.sub.n,G.sup.k and h.sub.n,G.sup.k
to flat levels .A-inverted.n. 2. REPEAT 3. FOR n = 1, . . . , N 4.
PSD allocation: each line determines and applies an optimized PSD
against the ghost line); 5.
.times..times..times..times..times..times..times..times..times..times-
..times..times..times. ##EQU00007## equation (2) and (2b))
.A-inverted.n, k; 6. central agent: Processing 7.
.times..times..times.>&.times..times.< ##EQU00008## 8.
.times..times..times.>&.times..times.<> ##EQU00009##
9. FOR n = 1, . . . , N 10. FOR k = 1, . . . , K 11.
.PHI..times..times..times..noteq. ##EQU00010## 12. IF
.PHI..sub.n.sup.k > L.sub.sup.sup.k 13. h.sub.n,G.sup.k =
.alpha.h.sub.n,G.sup.k; 14. else 15. h.sub.n,G.sup.k =
h.sub.n,G.sup.k/.alpha.; 16. central agent: Send h.sub.n,G.sup.k
.A-inverted.n, k 17. UNTIL convergence.
1. Initially, an initial ghost line PSD p.sub.n,G.sup.k and an
initial crosstalk gain h.sub.n,G.sup.k between a line and its
assigned ghost line are set to predetermined levels for all n, i.e.
for all modems.
These are the values that will influence the PSD allocation for all
users (see 4.) in the first iteration. The PSD allocation is done
in an optimization procedure against the ghost line: each user
should attempt to achieve its target, i.e. a minimum or maximum
rate for a given PSD or power limit, while doing the least damage
possible to the transmission of the ghost line. Known methods like
the dual decomposition approach disclosed in R. Cendrillon, W. Yu,
M. Moonen, J. Verlinden, and T. Bostoen, "Optimal Multi-user
Spectrum Management for Digital Subscriber Lines," in Proc. IEEE
International Conference ona Communications (ICC), Paris, 2004, pp.
1-5. are applicable but not exclusive.
The initial flat values of p.sub.n,G.sup.k (reference PSD) and
h.sub.n,G.sup.k (crosstalk gain between line and its ghost line) do
not represent real channel conditions and will be adjusted during
the execution of the method. .sigma..sub.n,G.sup.k is the reference
background noise. Bit loading for the ghost line may be calculated
as
.sigma..times. ##EQU00011##
Calculation of bit loading is a straight forward and already
practiced procedure [see all DMT-based DSL standards].
2. and 3. The following procedure is repeated for each
1.ltoreq.n.ltoreq.N, i.e. for all N modems, until the PSD
allocations converge, i.e. do not change considerably anymore.
4. Determine the PSD allocation against the current ghost line. If
no crosstalk information is yet received from the modems, the
initial ghost line is used. The PSD allocation can be done by using
either the RPM or the PMP strategy. Independent of that, the ghost
line should be able to achieve the highest possible rate. The
reason behind that is that this ensures the least crosstalk to the
rest of the network in all cases.
5. Each modem should have at its disposal a fixed estimation of its
background noise, which today's modem hardware easily can measure.
These values will be necessary for calculation of the following
relation
.times..times..times..times. ##EQU00012## which hereafter will be
referred to as Crosstalk Damage Ratio (CDR) of user n on tone k.
The CDR can be interpreted as the amount of crosstalk disturbance
user n experiences: b.sub.n,xt+bg.sup.k is bit loading when noise
is crosstalk plus background noise and b.sub.n,bg.sup.k is bit
loading when there would be only background noise. Since the
possible bit loading considering crosstalk is always lower with
crosstalk than without,
0.ltoreq.b.sub.n,xt+bg.sup.k.ltoreq.b.sub.n,bg.sup.k. As a
consequence CDR.sub.n.sup.k.epsilon.[0,1]. There are two extreme
cases, i.e. when CDR=0 and CDR=1. When CDR=0, crosstalk has no
impact on the achievable bit load and is therefore low (at least
compared to the background noise). When CDR=1, there is a lot
crosstalk impact, up to the point, where information transmission
is not possible anymore. As said, all other cases lie in between.
This is also indicated in FIG. 3a and is the reason why CDR can be
used as a crosstalk representing quality measure. From experience
in numerical simulations, it is known that every good DSM solution
should have the crosstalk damage ratio as low as possible. With the
sequence of iteration the central agent will suggest new values for
ghost line parameters so that rather low CDR values are
provided.
According to this embodiment the modems can also report to the
central agent a value that gives information about the amount of
channel resources necessary to be used compared to the ghost line.
The value w.sub.n, as defined in conjunction with equation (2) or
(2b) can be used, but other qualities such as waterlevel-based
qualities in water filling based approaches are suitable too. This
value, which also ranges from 0 to 1, can be interpreted as the
amount of priority user n needs to fulfil its rate requirements in
the competing optimization with the ghost line.
To illustrate what a demanding line is, consider the scenario on
FIG. 1. Such a scenario has a rate region such as that in FIG. 4.
The dotted line denotes a case when there is no crosstalk among
users. The full line denotes a hypothetical rate region for such a
scenario, in which the increase in the data rate of one user often
implies the decrease of rate on the other user. The points in which
the full line touches the R.sub.RT and R.sub.CO axis imply that
only one user is transmitting, i.e., these points represent
single-user points, in which the network is utilized by only one of
the users (in the figure the points (R.sub.RT.sup.max,0) and
(0,R.sub.CO.sup.max)). The same relationship is true for a line and
its ghost line. The points (R.sub.line.sup.max,0) and
(0,R.sub.Ghostline.sup.max) go along with w=1 and w=0, all other
points will correspond to a unique w.epsilon.(0,1) in the solution
of for example equation 2. Therefore, a direct relation between a
certain minimum rate requirement and w can be established and from
the weight or priority, the demanding target can be determined
based on the argument of w. Accordingly, the distance of a working
point to either of these points can be interpreted as an indicator
of how much emphasis during the optimization must be put on the
corresponding line.
After all w.sub.n's have been sent (only one w.sub.n per modem),
the central agent will choose the limits of the allowed crosstalk
range shown in FIG. 3b). The maximum value of w.sub.n, n>1, may
be generally set to a function of L.sub.sup and the minimum value
can be set to a function of L.sub.inf as well. Fixed limits are
also possible if no reliable information of the type of w is
available. The choice of limits for was indicated in conjunction
with FIG. 4 is reasonable: if one user (i.e. modem) sends a large
w.sub.n, it means that it has demanding requirements, which in turn
means that this user is allowed to emit more crosstalk to other
users and vice versa.
6.-8. The central agent attributes allowed and forbidden zones in
the CDR line for each particular scenario as shown in FIG. 3a. As
depicted in FIG. 3b, this division is according to this embodiment
characterized by a superior and an inferior value on the allowed
area, L.sub.sup and L.sub.inf, respectively. It should be noted
that only a superior limit may be used. This decision on allowed
and forbidden areas can also be smooth in terms of a weighted area
of acceptance, see FIG. 3c, as opposed to the "hard" decision as
indicated in FIG. 3b. For example, as stated above it is possible
to make the limits L.sub.sup or L.sub.inf soft, i.e. a factor or
weight is introduced reflecting the degree of acceptable
conditions, as indicated in FIG. 3c, in which the tones indicating
the different areas would gently turn into a darker or lighter
tone. This factor is used to put a weight on the resulting changes,
leading either to further increased or decreased changes. The
alternative of hard decisions as shown in FIG. 3b is considered
below. As said and in accordance with this embodiment, the limits
L.sub.sup and L.sub.inf respectively may be functions of w.sub.n.
The allowed range may also be a function of the CDR.
9.-11. After the choice of the allowed area, the method now
proceeds to adjustments of the ghost line parameter. The only ghost
line parameter that is changed is the crosstalk gain
h.sub.g,n.sup.k. Therefore, a decision variable .PHI..sub.n.sup.k
is calculated for every user and tone as a function of the reported
CDRs. It should be noted that the decision variable may also be
calculated as a function of the reported qualities w.sub.n in
addition to the reported CDRs as in line 11 in Table 3. A linear
combination of w.sub.n and the CDRs is suitable as found by
simulations, but a non-linear dependency can be applied, too. In
any case, emphasis should be given to the most damaged victim. It
is reasonable to consider that, since if the most damaged victim is
protected, then all other users are also protected. The calculation
of .PHI..sub.n.sup.k involves an additional weight matrixI, which
has the form of
##EQU00013## in which each element is either 0 or 1 (i.e.,
i.sub.n,j.epsilon.{0,1}). (i.sub.3,1 is the disturbance of the
third transmitter to the first receiver). If i.sub.n,j=0 implies
that user n does not interfere with user j. If i.sub.n,j=1 then it
is assumed that there is reasonable interference. This is in
principle the only necessary crosstalk channel information, which
implies that frequent channel measurements can be avoided. The I
matrix can be derived by primitive inspection of network topology,
since either there is considerable crosstalk between two specific
lines assumed or not. A further refinement of the interference
indicating matrix I by allowing values in between 0 and 1, assuming
different levels of channel knowledge, is possible to further
improve the quality of the results.
12.-15. The method then proceeds to the adjustment of the ghost
line parameter h.sub.n,G.sup.k. Three situations are possible: 1)
if the decision variable D is inside an allowed area then the
crosstalk user n causes to other users is within a desired margin
and no changes need to be done; 2) if .PHI..sub.n.sup.k is greater
than a L.sub.sup, then user n causes too excessive crosstalk to
others on this particular tone, which means that h.sub.n,G.sup.k
should be increased. This will result in a reduced interference
emission after the next iteration, since user n will be more
careful in allocating power on this tone to allow the ghost line
achieving a maximum rate (see also the description in step 4.); and
3) if .PHI..sub.n.sup.k is smaller than a L.sub.inf, then user n
causes no significant crosstalk for other users on this tone and
h.sub.n,G.sup.k can be decreased. Thus, on the next iteration user
n will have more freedom to allocate power on this tone. The way
how h.sub.n,G.sup.k is increased/decreased just affects the speed
of convergence and can be done by fixed or dynamical in-/decrements
or by a simple multiplication (or division) with a factor
.alpha..
16. After adjustment of the ghost line crosstalk gains for each
line, the central agent can now provide modems with the new
parameters. The modems can now allocate PSD again and send the
CDR's and w.sub.n's back to the central agent.
The method according to the invention is illustrated in the
flowchart of FIG. 5.
501 Allocate at the modems an initial power to the lines that are
optimized against a respective initial ghost line.
502. Transmit from the modems to the central agent of the DSL
network a measure (CDR) indicating the impact of crosstalk.
503. Calculate based on the received measure (CDR) indicating the
impact of crosstalk a decision variable (1) for the first modem and
ghost line parameters for the first modem based on the decision
variable (4)) for the first modem.
504. Report the calculated ghost line parameters (G) to the modem
of the first line.
505. Update the power to be allocated to the first line based on
the received updated ghost line parameters (G and continue with
step 502.
According to an embodiment of the invention, step 503 comprises the
further steps:
506. Determine an allowed crosstalk range (Lsup, Linf) for the
decision variable .PHI. as a function of the measure (CDR or CDR
and w).
507. Maintain previously determined ghost line parameters when the
estimated crosstalk-representing level is within the allowed
crosstalk range.
508. Increase a coupling between the ghost line and the first line
when the decision variable .PHI. is above the allowed crosstalk
range.
509. Decrease a coupling between the ghost line and the first line
when the decision variable .PHI. is below the allowed crosstalk
range.
To compare the performance of the proposed method with other
solutions the scenario in FIG. 6 was simulated. A Central Office
(CO) and three Remote Terminals (RT) transmitting in downstream
ADSL are involved in the scenario. Due to this near-far scenario,
the CO-downstream transmission is subject to heavy levels of
crosstalk and should be protected. Minimum rates for RT.sub.1 and
RT.sub.2 were set to 2 and 2.25 Mbps respectively, while the
minimum rate of RT.sub.3 ranged from 3 to 8 Mbps in each
simulation. For each situation, it was observed how many Mbps are
possible to provide to the CO user with BER of 10.sup.-7 and noise
margin of 12 dB. Three different optimization methods have their
rate regions as depicted in FIG. 7. ISB, the iterative version of
OSB which is an optimal but fully centralized solution, performs
better however with increased demands on complexity and channel
knowledge. For ASB the reference line was set to the crosstalk
characteristics between a line and the CO operated line (the upper
one in FIG. 6) in contrast to the present invention where each line
has a ghost line, which serves as an individual reference. It is
seen that among the three methods, ASB is the one with worst
performance, i.e. it results in the most restricted rate region.
The most outer line is said to be optimal and as such accepted in
the community. ISB is the most inner line and BLIND is the middle
line. In simple word, performance of a working point is better the
nearer it is to the most outer line.
It is worth to emphasize that both these method, ISB and ASB,
assume perfect channel knowledge.
The proposed method performs almost as good as the optimal one
(OSB/ISB, which is generally accepted to be optimal) but with
considerably less complexity and channel knowledge. It is seen that
the proposed solution achieves the best trade off among the most
important aspects for a practical DSM solution in terms of
performance, complexity, centralization and required system
knowledge.
Further, the method according to the present invention is
applicable for the general class of multi-carrier transmission
systems.
* * * * *